Despite intensified control efforts, malaria remains a major public health problem in many regions of the world. Recently, Southeast Asia has experienced resurgence of vivax malaria, and our knowledge of malaria epidemiology in this region is particularly scarce. Unlike the predominant Anopheles gambiae species complex transmitting Plasmodium falciparum in Africa, vectorial system in Southeast Asia includes multiple vector species and species complexes. Sympatric existence of P. vivax and P. falciparum further complicates malaria transmission dynamics. Because vector species differ in their susceptibility to different malaria parasite species and strains, changes occurring in the vector community will inevitably cause changes in the parasite population structure. To address the potential role of vector species composition changes in malaria epidemiology in eastern Thailand, we have selected Sa Kaeo province, which has experienced recent outbreaks of vivax malaria. Accompanying the rise of vivax malaria to its current predominant status in recent years, there has been an increase in abundance of the potential vectors Anopheles barbirostris group and a corresponding decline or disappearance of other major malaria vectors in this area. Based on these observations, we hypothesize that habitat changes associated with the expansion of agricultural fields and increased deforestation in Sa Kaeo may have significantly altered the vector community structure, which in turn may be responsible for the shift of P. vivax as the dominant malaria parasite species in the area. Accordingly, we will 1) examine the effects of environmental changes on the community structure of malaria vectors;and 2) determine the vectorial potential of An. barbirostris group for different parasite species and strains. This study will provide insights into the epidemiology of vivax malaria, and facilitate more targeted control of the vectors and disease. PUBLIC HEALTH RELEVANCE Malaria is a major public health problem in many nations. In order to understand the malaria epidemiology, we will study how environmental changes lead to vector community changes, which will affect malaria transmission, parasite species prevalence and genetic structure of parasite populations.